Question: The scale on a map is 8cm : 1km. If the distance between two cities is 4km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 8cm on the map corresponds to an actual distance of 1km. An actual distance of 4km is the same as 4 $\cdot$ 1km. The distance between the two cities on the map is 4 $\cdot$ 8cm, or 32cm.